1. Field of the Invention
The present invention relates to magnetrons, and more particularly, to specially designed anodes that enable higher frequencies to be generated by using frequency harmonics.
2. Description of Related Art
Magnetrons are known in the art and operate to convert DC electrical current to Radio Frequency (RF) power. FIG. 1 illustrates the basic operation of a class of magnetron known in the art as a slot-and-hole anode type magnetron. A heated cathode 102 acts as a source of electrons. The cathode passes through a central cylindrical cavity 106 passing through the anode 112. The anode 112 is formed from a conductive metal such as copper. A DC electric field is created by applying a DC voltage (not shown) between the anode 112 and the cathode 102; in the absence of a magnetic field this would cause electrons to travel in the radial direction 104 from the negatively charged cathode toward the anode. In the slot-and-hole type anode depicted, an even number of resonant cavities are formed by a rectangular opening (“slot”) 116 connected to a circular opening (“hole”) 114 with each slot forming a connection between the central cylindrical cavity 106 and the hole. A magnetic field 122 is applied in a direction perpendicular to the electric field (pointing out of the page in the figure). The magnetic field acts on the radially accelerating electrons causing the electron's movement path to curve. As the electrons pass through the magnetic field, electrons passing by a slot opening give up some energy, and the resonant cavities begin to oscillate at a natural resonant frequency determined by the geometry of the cavities. The metal walls of each resonant cavity facing the central cavity 120 (“vanes”), interact with the passing electrons and build up localized charge distributions that change with the resonant cavity oscillations.
FIG. 2 shows an instantaneous view of the RF electric field of a magnetron operating in the π mode. The term π mode refers to the phase difference in radians of the RF electric field between adjacent vanes in the magnetron anode. Arrowed lines 202 show the direction of the RF electric field vectors at a particular half cycle maximum of the magnetron operation. Two shorting rings 207 connect to alternating vanes at contact points 209, the inner ring connects to “even” vanes while the outer ring connects to “odd” vanes. The alternating circumferential field maximums 204 interact with the electrons curling through the magnetic field, and allow a sustained oscillation at the operating frequency of the device.
Because the operating frequency of magnetron operation depends on the dimensions of the anode and resonant cavities, magnetrons typical of the prior art operating at higher frequencies decrease in size as operating frequencies increase. The smaller vanes associated with smaller cavities are unable to remove heat as quickly. Smaller central cavities also require a greater magnetic field strength to properly divert the electrons emitted from the cathode over a shorter traveling distance between anode and cathode. Since output power also depends on the DC electric field established between the cathode and anode, the corresponding reduced central cavity dimensions increase the likelihood of breakdown voltage gradients. Additionally, cathode loading becomes a limiting factor as frequency increases. Due to these limitations, a larger magnetron anode capable of operating at higher frequencies is desirable.